Preheat the oven to gas mark 5/375oF/190oC and line a 8 x 8 baking tin with baking paper. Make the filling by adding the chopped apple, granulated sugar, cornflour, lemon juice, cinammon, nutmeg and vanilla extract to a medium bowl then combine. Set aside.
In a stand mixer bowl or a mixing bowl combine the plain flour, oats, granulated sugar, brown sugar and salt then add the butter to a microwavable bowl and melt in the microwave using 35 second increments. Pour the melted butter into the bowl with the dry ingredients and using the paddle attachment on a stand mixer or a hand mixer combine the butter with the dry ingredients until they are well combined..
Pour just over half of the crumble mixture into the baking tin then evenly spread and press the mixture into the bottom of the baking tin. Ensure it is tight and compact. Spread the apple filling evenly on top of the pressed down crumble. Now take the rest of the crumble, sprinkle it on top of the apples and slightly press down. Optionally, you can sprinkle a ½ teaspoon of granulated sugar on top. Bake for 50 minutes or until golden brown. Allow to cool before cutting into 16 bars.